How can we improve WHMCS?

My company would like to be able to sell a subdomain to our customers but have it as a product addon. This would need to be able to be verified so a customer can easily choose a custom subdomain without conflict of an existing subdomain. This would appear as a text-box and allow a customer to enter a custom subdomain.oursubdomain and tell the customer if it is available. This would also need to either only appear if the customer selects yes to the custom domain or apply a flat fee that would be able to be added to the total monthly price.